Masahiro Kadosawa, Masaaki Nakamura, Yukinori Ohta, Satoshi Nishimoto
We investigate the ground-state magnetic structure of the spin- 1 / 2 XXZ antiferromagnet on the triangular lattice in the easy-axis regime using the density-matrix renormalization group. By applying spiral boundary conditions, we exactly map finite L × L clusters onto one-dimensional chains while avoiding the spatial anisotropy inherent in cylindrical geometries. From symmetry-broken local magnetization profiles, we extract the three-sublattice moments and track their evolution with anisotropy. At the isotropic point, we obtain a positive sublattice moment of 0.217(3), consistent with previous numerical estimates. In the easy-axis regime ( Δ = J z / J ⊥ > 1 ), the ordered moments remain close to a Y-like zero-magnetization three-sublattice state, whose z -component pattern is of the form ( 2 m , − m , − m ) , over a broad range of Δ . Extrapolation in 1 / Δ shows that the positive sublattice moment stays well below the classical saturation value 1 / 2 , approaching 0.419(7) as Δ → ∞ , while the magnitude of the negative sublattice moment approaches 0.209(4). We further compare the energies of the Y state and the up-down-down state and find that the Y state is favored at zero field. Independent thermodynamic-limit energy calculations, performed without assuming any particular ordered pattern, yield an energy consistent with the Y-state solution. These results show that the easy-axis ground state does not simply cross over to a trivially saturated collinear Ising state, but instead remains a nontrivial three-sublattice ordered state selected from the macroscopically degenerate Ising manifold by quantum fluctuations.